1. Field of the Invention
This invention relates to a plating coating having a non-homogeneous distribution of a dispersed substance, in particular, to such a plating coating having a conspicuous distribution of the dispersed substance changed at intervals or continuously in the direction in which the plating coating is formed, especially suitable for nickel plating of the interior surfaces of cylinders of internal combustion engine blocks.
2. Background of the Art
A particular plating in which a dispersed substance-forming material is used, so-called composite plating, is hitherto known as one of the plating methods which can provide a plating layer having excellent lubricity and frictional properties. Composite plating is conducted advantageously in plating the interior surfaces of cylinders of internal combustion engine blocks.
In composite plating, various requirements for plating layers cannot be satisfied in some cases. For example, adhesion between a composite plating layer and a work is impaired when using one composite plating layer in which a dispersed substance is formed at an amount sufficient to obtain satisfactory lubricity or frictional properties. To improve adhesion between a composite plating layer and a work, an attempt is known, in which a work and an electrode are immersed in a plating liquid containing a dispersed substance-forming material, which is stored in a tank, a constant voltage is impressed therebetween at a low electric current density (e.g., 100 A/dm.sup.2), and then a constant voltage is impressed therebetween at an electric current density (e.g., 200-300 A/dm.sup.2) higher than the initial density, whereby the first plating layer contains a dispersed substance 0.5% by weight less than that of the second plating layer which contains 1.5-3.5% by weight (fluctuation due to the dispersion of the contents of the plating liquid) so as to indirectly strengthen adhesion between the second plating layer and the surface of a work. However, based on the above control, the maximum difference between the first plating layer and the second plating layer in the amount of a dispersed substance would be approximately only 0.5%, and it is impossible to enlarge the difference more than such since it is not in practice to further decrease the electric current density to decrease the amount of a dispersed substance in the first plating coating due to too slow a deposition speed, and it is also not in practice to further increase the electric current density to increase the amount of a dispersed substance in the second plating layer due to serious impairment of the quality of the layer (metal hydroxide or oxide, not metal itself, tends to be educed on a work). Thus, the first plating layer cannot be sufficient to improve adhesion of the second plating layer due to similar dispersed substance contents between the first and second plating layers, and thus the thickness of the first plating layer is limited to 1 .mu.m or less since no further effects or even adverse effects are expected. Hitherto, a composite plating layer containing a dispersed substance and a plating coating composed of a base plating layer containing significantly less dispersed substance, which are stacked on the surface of a work and which are formed from the same metal base plating liquid, has not been achieved.
As described above, immersing a work in a plating bath to plate the work is commonly conducted. In order to improve productivity of the common plating treatment, a technology to speed up plating processes by impressing a voltage between the surface of a work to be plated and an electrode while permitting a plating liquid to flow therebetween has been recently developed. However, even if the high speed plating method is adopted, plural plating layers composed of a base plating layer and a composite plating layer has not been successful.